In the area of recreational vehicles such as travel trailers and motor homes, and other on-highway and off-highway vehicles, there is a need for leveling when these vehicles are parked for use. Recreational vehicles and campers are usually parked in campsites where the parking area is not always level. Off-highway vehicles may be parked in much more uneven terrain. Various systems are found in the prior art relating to leveling such vehicles.
These systems include at least a lift mechanism and a level sensing device. Generally, the designs use mechanical, electromechanical, or hydraulic jack jacks and level sensors for leveling the vehicle. The devices are strategically mounted to the underbody or chassis of the vehicle to achieve the leveling quickly and efficiently. The number of jacks and level sensors used in each application is dependent on the size and weight of the vehicle being supported, among other things. In one example, a recreational vehicle is equipped with four jacks in the form of hydraulic cylinders mounted to the vehicle frame. Two jacks are located adjacent to the rear of the vehicle and two jacks are located adjacent to the front of the vehicle. In another example, a vehicle leveling system has three jacks, two of which are located at the rearward end of the vehicle and one of which is located at the forward end of the vehicle. Other systems may have more or fewer jacks. Such systems are described, for example, in U.S. Pat. Nos. 5,176,391; 5,312,119; 5,628,521; 5,676,385; 5,901,969; 5,913,525; 5,890,721; 5,915,700; 6,848,693; 7,025,178 and 7,815,200.
Such systems may employ jacks that are hydraulically powered to extend and retract. Sensing that the jacks have fully retracted is sensed by an elevated hydraulic pressure in the system, sensing the pressure applied to the retract side of the jacks. That works well when the viscosity of the hydraulic fluid is in a normal range and there are no other obstructions in the hydraulic system, such as dirt or a pinched hose, or in extreme cold. Under those conditions, due to the resistance of the fluid on the extend side of the jacks to return to tank, an elevated pressure may develop on the retract side that is sufficient to trip the pressure sensor and give a false positive indication that the jacks are fully retracted. With that indication, the driver may attempt to move the vehicle, even with the jacks not fully retracted, which is an undesirable condition.
The present invention is aimed at assuring full retraction of the jacks even under such conditions.